Photoacoustic microscopy of lipids at 1.2 and 1.7 μm using a pulsed supercontinuum laser

Photoacoustic microscopy (PAM) is promising for label-free histology of lipid-rich tissue. Lipids have optical absorption peaks at 1.2 and 1.7 μm, but these wavelengths are typically produced with expensive optical parametric oscillator (OPO) lasers. Photonic crystal fibers (PCFs) have nonlinear optical properties that enable a single-wavelength laser to produce an ultra-broadband supercontinuum spanning the visible and near-infrared. In this paper, we demonstrate near-infrared PAM of lipids using a supercontinuum PCF laser. Our system starts with a 1047 nm Q-switched Nd:YLF laser producing 14 ns pulses with 100 uJ of energy at a 2.5 kHz repetition rate. These pulses are sent through 30 meters of polarization-maintaining large-mode area photonic crystal fiber (LMA-PCF) with a 10 μm core diameter. The desired PAM wavelength is selected with a dielectric band-pass filter (50 nm bandwidth) and focused onto the sample with a 4X microscope objective. The laser pulse energy at 1047, 1100, 1225, 1325, 1600, and 1714 nm are 6.81, 6.43, 1.07, 1.23, 0.76, and 0.32 μJ, respectively. Imaging experiments are performed on a lipid phantom (butter in clear gelatin) and Drosophila larva. Lipid contrast is 12 dB higher at 1714 nm compared to 1225 nm, despite the much lower pulse energy at 1714 nm. To our knowledge, this is the first demonstration of a supercontinuum laser for near-infrared PAM of lipids. We believe our system's wavelength flexibility make it promising for label-free histology of lipid-rich tissue.